1. Field of the Invention
The present invention relates to the field of reflector antennas, and more particularly, to a reflector antenna which includes a plurality of frequency selective or polarization sensitive structures to provide a plurality of antenna patterns from a single support structure.
2. Description of the Prior Art
Reflector antennas are frequently used on spacecrafts to provide communication links with the ground or other spacecrafts. A single spacecraft will typically house multiple antennas to provide multiple communication links. These multiple antennas on a single spacecraft typically operate at different frequencies or polarizations to lower crosstalk and interference between antennas.
One method of providing multiple frequencies and multiple communication capabilities on a single spacecraft is to provide multiple reflector antennas, one for each desired frequency of operation. Although this method provides good isolation between antennas, it requires a large amount of space on a spacecraft, is expensive and extracts a weight penalty.
A second method of providing multiple frequencies and multiple communication capabilities on a single spacecraft is to provide a single reflector antenna having multiple feeds, each feed radiating a separate RF frequency or polarization. One feed is placed at the focal point of the reflector while the other feeds are located as near the focal point as practical. This results in a loss of signal strength for the unfocused feeds and may require a larger reflector to compensate for the losses. A larger reflector requires more space on the spacecraft and provides an antenna pattern with a narrower beamwidth, which may be undesirable.
A third method of providing multiple frequencies and multiple communication capabilities on a single spacecraft is to utilize a frequency sensitive structure, also known as a dichroic structure, as the subreflector in a cassegrain type reflector antenna. A cassegrain type reflector antenna has a main reflector and a smaller subreflector. The dichroic subreflector is hyperbolic in shape and has two focal points, one located on each side of the subreflector. The subreflector is placed between the main reflector and the focal point of the main reflector with the convex side of the subreflector facing the main reflector. The focal point on the concave side of the subreflector is placed at the focal point of the main reflector and a first feed, radiating a first RF signal at a first frequency, is placed at this focal point. The dichroic subreflector is configured to pass the first RF signal through the subreflector such that the first RF signal will be incident on the main reflector and generate a first antenna pattern at a first frequency.
A second feed, radiating a second RF signal at a second frequency, is placed at the focal point on the convex side of the subreflector. The dichroic subreflector is configured to reflect the second RF signal and redirect it towards the main reflector such that the second RF signal will be incident on the main reflector and create a second antenna pattern at a second frequency. In this way, a single reflector can provide antenna patterns at two separate frequencies. This scheme, however, is limited to combining two antennas into a single structure. In addition, the size of the reflector typically determines the gain and beamwidth of the antenna pattern and the focal axis determines the location of the antenna pattern. Using a single main reflector with a dichroic subreflector typically results in the first and second antenna patterns having the same gain-beamwidth product and the same location which may be undesirable. A subreflector can also add a level of complexity to the antenna and provide antenna blockage that may be undesirable.
A need exists to have a single reflector apparatus with multiple focal points. This would allow a single spacecraft to carry the weight and expense of one reflector apparatus while having the ability to provide communication links with multiple communication stations or vehicles.